Packaged Rooftop Air Conditioners

The majority of US commercial floor space is cooled by self-contained, packaged air-conditioning and heat pump units, most of which sit on rooftops (Figure 1). These rooftop units (RTUs), also called unitary air conditioners or simply “packaged units,” are mass-produced machines that include cooling equipment, air-handling fans, and sometimes gas or electric heating equipment. RTUs are available in sizes ranging from 1 ton to more than 100 tons of air-conditioning capacity (1 ton of cooling capacity will remove 12,000 Btu of heat per hour).

Figure 1: Up on the roof

Rooftop units are the workhorses of commercial air conditioning and are used widely in industrial facilities as well.

The compressor is the main power consumer in an RTU, accounting for about 80 percent of the peak power; the supply fan and condenser fan equally consume the remaining 20 percent (Figure 2). However, because supply fans are used to provide ventilation and deliver outside air to maintain indoor-air quality even when the compressor is not in use, the compressor’s annual energy usage typically accounts for only about 55 percent of total energy use, while fans account for the remaining 45 percent.

Efficiency is the key word when it comes to RTUs, but it can take many forms. There are multiple opportunities to deploy efficient RTUs in new construction, replacement, and retrofit applications.

High efficiency. RTUs are available with many different levels of efficiency. Several national efforts are under way to increase the efficiency of new products and to develop and deploy retrofit options that will increase the efficiency of the large installed base. In the field, poor maintenance practices and the inability to control for part-load operating conditions have led to a generally low installed efficiency of RTUs. ASHRAE (the American Society of Heating, Refrigerating, and Air-Conditioning Engineers) and the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) evaluate RTU efficiency based on three primary performance metrics:

Energy-efficiency ratio (EER): The ratio of the rate of cooling (in Btu/hour) to the power input (in watts) at full-load conditions. The power draw includes all inputs to compressors, fan motors, and controls.

Seasonal energy-efficiency ratio (SEER): A seasonally adjusted rating based on representative residential loads. SEER applies only to RTUs with a cooling capacity of less than 65,000 Btu/hour.

Integrated energy-efficiency ratio (IEER): A measure that expresses overall operating efficiency on the basis of weighted operation at different part-load capacities.

EER is the rating of choice when determining which RTU will operate most efficiently during full-load conditions. SEER and IEER are better indicators of which RTU will use less energy over the course of the year or cooling season.

Federal minimum standards. The current ASHRAE 90.1-2010 Standard requires a minimum EER of 11.0 and a minimum IEER of 11.2 for the typical commercial RTU.

High-Performance RTU Challenge. In an effort to move the needle on efficiency improvements and help companies capture significant energy savings, in 2013 the US Department of Energy (DOE) issued a challenge to equipment manufacturers to produce RTUs designed to reduce energy use by more than 50 percent over the ASHRAE 90.1 Standard. For RTU equipment to qualify, it must meet the Consortium for Energy Efficiency’s (CEE’s) Tier 2 standards, which vary by equipment size and technology type. The required minimum IEER for RTU models entered for consideration in the challenge is 18.0.

In 2012, the Daikin Rebel was the first RTU to meet the DOE’s High-Performance RTU Challenge requirements, with an AHRI-certified IEER of 20.6. In 2013, Carrier’s WeatherExpert also met the challenge’s performance requirements with an IEER of 20.8. Other companies working to develop commercial RTU models that meet the high-performance standards include Lennox, Rheem, and 7AC Technologies.

Advanced RTU Campaign. In an effort similar to the High-Performance RTU Challenge, the DOE’s Office of Energy Efficiency and Renewable Energy’s (EERE’s) Better Buildings Alliance is leading the Advanced RTU Campaign to help motivate companies to adopt high-performance RTUs and retrofit control devices. The campaign provides an easy-to-use checklist and decision tree (PDF) to assist participants in comparing retrofit versus replacement options and evaluating performance and savings potentials for various applications. In addition to the CEE Tier 2 performance standards, this campaign requires that qualifying equipment use ASHRAE 90.1–compliant economizers and that standard maintenance practices are followed and enforced according to ASHRAE Standard 180. Verification of program compliance is based on an honor system and participant-provided information.

Efficient compressor controls. Most RTUs use efficient reciprocating compressors that have several control options. RTUs generally handle part-load conditions with simple on/off switches that stage compressors with programmable timers. As an alternative to completely shutting off the compressor, high-efficiency units offer either staged-capacity or variable-capacity compressor-control options to reduce compressor energy usage during part-load operation.

Efficient condenser options. Nearly all RTUs under 20 tons have air-cooled condensers, which are about 20 percent less efficient than the evaporative condensers used in larger units. The efficiency of air-cooled condensers can be improved by incorporating “microchannels” and other advanced heat-exchange technologies into the design to increase the rate of heat removal, but at an additional cost.

Efficient fan motors and controls. Fans are used to move air across the RTU’s condenser and evaporator. The airflow across the evaporator is also typically used as the supply air for the building. Although fan power use is a small fraction of compressor power use, fans can account for approximately 45 percent of annual energy use because they operate for many more hours than the compressor does. Most manufacturers also offer units with high-efficiency fan motors that increase EER, as well as variable-speed fans that improve IEER.

Economizers. An economizer is a dampered cabinet opening that draws in air from outdoors when the outside air is cooler than the temperature inside the building, thereby providing “free” cooling. Many codes, standards, and utility programs already require the use of economizers (including ASHRAE 90.1-2013). Economizers can reduce energy use by anywhere from 15 to 80 percent, depending on local conditions, and they are usually cost-effective given their relatively small added up-front cost. To ensure energy savings and proper operation, economizers should be checked regularly to be certain that they’re operating properly—the dampers can get stuck open, which leads to increased cooling loads as the economizer draws in outdoor air even when it’s warmer than the air inside.

Standard controls. Programmable digital controls, which offer flexible settings that can be tailored to the application, are increasingly available as standard equipment. A common example is a seven-day scheduler that consistently operates the RTU according to occupancy expectations and nighttime temperature setbacks. Digital controls are also easily tied into a central energy-management system for monitoring and control as part of an overall building control strategy. In addition, many new RTUs come ready to accept inputs from carbon-dioxide occupancy sensors. These can be used to implement demand-controlled ventilation, an energy-saving strategy that adjusts building ventilation as occupancy changes rather than assuming that the building is always fully occupied.

Retrofit controls. A number of retrofit control devices are now available as commercial products and can deliver significant energy savings, largely from the application of variable-speed fan controls to the majority of existing units that still operate at constant speed (Figure 3). One model (the Catalyst from Transformative Wave) allows for discrete setpoints at 40, 75, and 90 percent of full-load fan speed. In addition to the Catalyst, other available retrofit device models include the Enerfit V1 and the Bes-Tech Digi-RTU. Demonstrated energy savings from successful trials range anywhere from 25 to 70 percent, with the wide range due in large part to differences in the magnitude of energy waste in baseline operating conditions. Demonstrated simple payback periods on retrofit investments for commercial applications are approximately two years.

Figure 3: RTU retrofit controllers save energy

Retrofit controllers for rooftop units (RTUs) convert single-speed RTUs to variable-speed units, which can result in energy savings of as much as 70 percent. Fault detection and diagnostics, remote monitoring, and variable-speed condenser fan and compressor control are offered as additional features.

Fault detection and diagnostics. Over the past several years, growing emphasis has been placed on the need for fault detection and diagnostics (FDD), the ability to find and diagnose errors in RTU operation. Among the first commercialization efforts of equipment with FDD capabilities can be seen in the Catalyst retrofit controller, which currently includes anomaly detection and economizer failure alerts. The makers of the Catalyst have said that they intend to include several additional FDD features in the future, including condenser and evaporator coil fouling, excessively high or low refrigerant levels, liquid line restrictions, compressor valve leakage, and the presence of noncondensable gas.

Evaporator coils. RTUs normally use direct-expansion evaporator coils, in which air is blown over a fin-and-tube heat exchanger that carries the evaporating refrigerant. Electronically controlled expansion valves are the best available technology for ensuring that refrigerant is efficiently metered between the evaporator and condenser and maintained at the proper temperature and state for the operating conditions.

How to Make the Best Choice

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Whether you’re considering improving the efficiency of an existing RTU, replacing one, or planning for new construction, many options are available. We provide a starting point to help you identify and assess your HVAC needs so you can choose an RTU that offers optimal efficiency at a price you can afford.

Consider retrofitting your existing equipment. It can be difficult to justify the time and expense of replacing existing RTU equipment with advanced, high-efficiency models, and this challenge can lead to inefficient units that limp along year after year. Advanced RTU retrofit controllers are demonstrating very impressive energy savings—regularly as high as 60 to 70 percent in field demonstrations—but without the same level of up-front cost and equipment installation required for full replacement. With demonstrated two-year simple payback periods in commercial applications, advanced RTU retrofit controllers have become a very attractive option for securing persistent energy savings. For more details on previous demonstrations and evaluation results, see the Advanced RTU Campaign’s Case Studies & Guidance page.

Size equipment appropriately. An undersized unit won’t be able to provide sufficient cooling, but if a unit is oversized (the more frequent occurrence), it not only costs more but will lead to higher costs for associated ductwork and other auxiliaries. Operating costs may increase too, unless the unit is equipped with adjustable-speed drives, because oversized equipment spends more time at less-efficient part-load conditions.

In the past, attempts at remedying this issue called for “right-sizing” RTU equipment to avoid unnecessarily delivering excess cooling; this is less of an issue with advanced RTU models that have variable-speed control capabilities. In fact, field testing of the Daikin Rebel by researchers at the Western Cooling Efficiency Center (WCEC) in California found that, thanks to its variable-speed control capabilities, it was actually better to err on the side of over- rather than undersizing in order to achieve the greatest overall energy savings.

Identify high-efficiency models. AHRI is the main source of information about RTU product efficiency ratings. The organization maintains a Directory of Certified Product Performance (available in both print and electronic formats) on its web site that includes products from all AHRI member-manufacturers.

The CEE offers a program known as the High Efficiency Commercial Air Conditioning and Heat Pump (HECAC) Initiative. The initiative’s goal is to encourage the use of high-efficiency unitary central air-conditioning and heat pump equipment in commercial buildings (unitary equipment consists of both single-packaged units, which contain all major assemblies in one cabinet, and split systems, which have one or more of the major assemblies separate from the others). Utilities participating in the initiative use the CEE’s high-efficiency equipment specifications in their education and rebate programs. The CEE also maintains an easy-to-use directory of AHRI-verified equipment that lists RTUs with capacities less than 65,000 Btu/hour using both three-phase (commercial) and single-phase (residential) power.

The Energy Star program, which is jointly operated by the US Environmental Protection Agency and the DOE, establishes an efficiency specification above the federal standards. Equipment that meets these specifications is awarded the Energy Star label, which makes it easy for consumers to identify high-efficiency products. Energy Star’s Program Requirements for Light Commercial HVAC were set in 2002. Recently, the DOE’s Appliance Standards and Rulemaking Federal Advisory Committee proposed a new ruling, which is expected to go into effect in two phases. The first phase will increase RTU efficiency (IEER) by 10 percent in 2018 (in compliance with the ASHRAE 90.1-2013 Standard), and the second will add a 25 to 30 percent improvement in IEER for RTUs manufactured as of January 2023.

Evaluate high-efficiency models by performing a cost-effectiveness calculation. The cost-effectiveness of a high-efficiency RTU depends on several factors, including cooling loads, operating hours, and the local cost of electricity. The DOE and Pacific Northwest National Laboratory (PNNL) offer a Rooftop Unit Comparison Calculator that evaluates estimated lifetime energy costs between RTUs with various efficiency levels, sizes, and hours of operation.

Pay attention to design, commissioning, and maintenance. No matter what equipment you choose, it’s also important to make sure that the overall system is designed to be efficient (Figure 4), that it’s commissioned to operate as planned, and that it’s properly maintained. Attaching the RTU to a low-static-pressure air distribution system will reduce control problems, noise, and fan power requirements.

Figure 4: RTU components, designed for energy efficiency

The ideal packaged rooftop unit (RTU) is designed to maximize energy efficiency. Some available systems are built similarly to the one we’ve illustrated here, but many are not, or they have some but not all of these energy features. The RTU shown contains electric cooling and gas heating components.

Comprehensive testing, adjusting, and balancing of the installed unit, its controls, and the air distribution system will maximize installed efficiency and comfort. Conducting regular tune-ups, correcting refrigerant charge, cleaning and adjusting the system to correct airflow and improve heat transfer, verifying proper economizer operation, and repairing major duct leaks can all yield surprising energy savings at relatively low cost. TAB (testing, adjusting, and balancing) contractors; general HVAC service contractors; and contractors that use specialized diagnostic equipment can all be called on to perform RTU tune-ups and adjustments.

What’s on the Horizon?

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In the future, federal standards will likely drive RTU efficiency levels higher. To achieve higher efficiencies, more RTUs will be equipped with variable-speed and staged compressors for improved part-load operating performance. In addition, technologies such as evaporative cooling of the condenser and energy-recovery devices may begin to be widely incorporated into RTUs.

The California Energy Commission’s Public Interest Energy Research Program has developed sophisticated RTU designs through its Advanced Automated HVAC Fault Detections and Diagnostics Commercialization Program (PDF). A similar program, run by the DOE and PNNL, is under way to develop Smart Monitoring and Diagnostic Systems. These programs seek to develop FDD capabilities for HVAC systems as well as more fault-resistant HVAC equipment. They also promote working with manufacturers to implement these features into new products. By automatically notifying service personnel of broken or malfunctioning equipment as soon as problems occur, these advanced units will help prevent equipment from degrading too quickly and from operating at low efficiency levels.

Researchers have found that the annual net savings range is from $400 to $1,000 and the estimated simple payback period is less than one year. Greater savings are possible in hotter climates due to larger cooling requirements. The savings would also be greater for heat pumps because they operate year-round.

For further background information, AHRI offers downloadable versions of current unitary equipment standards on its website. It also offers a list of links to other efficiency-related and HVAC-related sites and organizations.

Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by E Source, nor does any content of the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of a service provider.